Complex composite particles and methods

ABSTRACT

A complex composite particle is made of a coal dust and binder composite that is pyrolyzed. Constituent portions of the composite react together causing the particles to increase in density and reduce in size during pyrolyzation, yielding a particle suitable for use as a proppant or in a composite structure.

CROSS RELATED APPLICATIONS

This application is a 371 U.S. national phase application ofPCT/US2017/058616 filed Oct. 26, 2017 which claims priority of thefiling date of U.S. Provisional Appl. No. 62/413,385, which was filed onOct. 26, 2016, the entirety of which is hereby incorporated by referenceherein, except for paragraph [0008] of the provisional application.

FIELD OF THE INVENTION

The field relates to composite materials, especially to in situcomposites made using coal dust and coal particles.

BACKGROUND

Experts in the field did not think that coal dust or small coalparticles could be used to make composite particles and compositearticles. The impurities in every source of coal, which is a fossil fueldug from the ground, is legendary. Every source of coal is comprised ofcarbonaceous material and many impurities, often hundreds of organic andinorganic substances are found in coal, when it is analyzed. O. Charonet al., “Variation in Coal Composition: a Computational Approach toStudy the Mineral Composition of Individual Coal Particles,” shows anexample of a composition of one source of coal, Upper Freeport raw coal,and tries to use a computational model to predict the fly ash evolutionduring pulverized coal combustion, which depends on the amount,composition and spatial distribution of the inorganic matter withinindividual particles, using computer controlled scanning electronmicroscopy (CCSEM) to measure mineral distributions in particles asinput to the computation model. The model only accounts for five mineralspecies: quartz, kaolinite, illite, mixed silicates, and pyrite. Eventhough other minerals exist, these are considered the main constituentsaffecting fly ash evolution (and were the ones available from CCSEManalysis). Other ways of analyzing coal is by Electron Spectroscopy forChemical Analysis (ESCA) and Fourier Transform Infrared Spectroscopy(FTIR). Cara L., et al., “ESCA and FTIR Studies of Bituminous Coal,”discloses a study of surface chemistry of coal powder showing elementscarbon, oxygen, nitrogen, sulfur under various storage conditions.Sulfur was found on the surface of some but not all of the samples.Other elements were detected including: aluminum, silicon, sodium, ironand potassium, but the iron and potassium elements were found only onthe surface of one of the raw samples. The sulfur was reportedly foundin inorganic and organic species. While a great deal of research hasbeen done to determine compositions of a few types of coal, these typesof coal have been analyzed for the effects of composition on use of thecoal as a fuel and not as a structural material.

Experts in the field of composites were convinced that coal dust wouldnot provide consistent results comparable to modern composite materials,which are engineered using sources of consistent fillers and compatiblepolymers and resins. U.S. Pat. No. 2,638,456 was filed in 1949 andissued on May 12, 1953, and it successfully incorporated anthraciteparticles, the purest form of coal, as a filler in a rubber matrix forplastic cases. However, it used a standard process at the time formixing anthracite and synthetic rubber. Similarly, U.S. Pat. No.3,915,906 issued in the 1970's and used coal powder mixed with apolymer, and optionally reinforcing fibers, to make gaskets. Again, thepolymer was conventional mixed with the powder and formed as a sheet orthe like.

Some other references in the literature to “coal” as a filler arereferring to coal as a raw ingredient that is processed by cracking toproduce coal tar, graphite, coke or the like. Cracking substantiallychanges the chemistry and nature of coal, and the definition of coal,herein, does not include products produced by cracking or other hightemperature processing, dissolution, liquification or the like. Instead,coal refers to raw coal, which may be processed by milling to reduce thesize of coal particles, sorting into particular particle sizes anddrying, to remove water and/or volatiles, which does not change,substantially, the chemical nature of the coal.

U.S. Pat. No. 8,961,840, the disclosure of which is incorporated byreference herein in its entirety, discloses a process of makingspherical ceramic particles utilizing a process that dries particlesformed from a polymer selected to form a ceramic when pyrolyzed. Theseceramic particles are referred to as polymer derived ceramic particles.

A proppant is a solid material, typically sand, treated sand or man-madeceramic materials, designed to keep an induced hydraulic fracture open,during or following a fracturing treatment. The proppant must resistcrushing and must have sufficient permeability through or around theproppant particles for extraction of gas. Proppants must allow gas underhigh pressure to escape through interstitial spaces between particles.There is a trade off between size of interstitial spaces and strength.Proppants must have sufficient mechanical strength to withstand closurestresses in order to hold fractures open after fracturing fluid iswithdrawn and pressure released. While large proppant particle sizeshave greater permeability than smaller particle sizes, at low closurestresses, higher closer stresses cause proppants with large particlesizes to fail mechanically. Crushing produces very fine particulates,called fines, at high closure stresses. Smaller particle size forproppants is preferred for higher closure stresses. Sand, ceramics andsintered bauxite particles are known to have been used as proppants, andthere are benefits and disadvantages using any of the known proppants.ISO 13503-2:2006 is a standard for measuring the properties of proppantsthat has been adopted by industry. Unless otherwise specified,properties of proppants disclosed are determined according to thisstandard.

One trade-off is strength and density. Increased strength is known tocorn, with increased density. Increased density typically requireshigher pressures, costs and other complications. Furthermore, lowdensity proppants are preferred over higher density proppants, because alower density (less than 2.5 grams per cubic centimeter) reduces pumpingpressures, increases fluid velocities and keeps particles in suspensionwithout settling out, longer. Proppant geometry is also important, withspherical particle shapes and round particle shapes being preferred overnon-spherical and particles with angular features. Proppant choice andcost impacts output rate, recovery and return on investment of a well.One significant cost is transportation costs from supplier to site, andtransportation cost is directly related to density, also.

Coal dust may be formed by coal mining operations and by deliberatelycrushing coal and/or milling the coal dust into finer and finerparticles. The particle size of coal dust is typically in a range from 1to 100 microns, although any size of coal particle may be formed bycrushing, milling and sorting of coal dust. Herein, the definition ofcoal dust includes dust made from coal deposits and synthetic carbondust, such as synthetic graphite, to the extent that such syntheticcarbon dust is modified to have the same benefits as coal dust. Forexample, additives may be added to synthetic carbon dust to cause it tohave the same characteristics during pyrolyzation as the dust producedfrom mined coal dust or mined coal dust that is preheated to drive offlow-temperature volatile gases and/or some of the water. Any source ofcarbon dust that has been modified to substantially perform as coal dustperforms is referred to herein as coal dust. Synthetic graphite may beprovided having particles sizes of 1 to 10 microns with particle sizedistributions of 10 to 50 percent of the particle size, depending on theparticle size selected. Additives such as organic compounds andhydrocarbons may be added to synthetic graphite to make syntheticgraphite behave more like coal dust. Surprisingly, synthetic graphite,when added to a polymer derived ceramic material fails to provideparticles and composite articles having the same characteristics, whenprocessed identically, as coal dust. In one example, coal dust waspre-pyrolyzed to drive off water and organic compounds from the coaldust, leaving a powdered carbon dust or a gravely mixture of carbon,depending on the source of the coal, which was then milled to a powderycarbon dust. The pre-pyrolyzed coal dust, which was pyrolyzed in asubstantially non-oxidizing atmosphere, produced a carbon dust thatfailed to produce particles and composite articles as un-pyrolyzed coaldust. However, pre-heated coal dust, which was preheated at atemperature less than 400 degrees centigrade, driving off lowtemperature volatile organic compounds and some water, performed thesame or similarly when used in composite particles and articles as theas-received coal dust. Coal is abundantly available and costs less than$0.02 center per pound and is comparatively easy to crush into a dustand to grade into various sieve sizes. However, due to coal dustsvarying compositions, depending on the source and type of the coal, ithas not been considered as a source for production of modern compositematerials. Surprisingly, the use of coal dust, which contains a numberof volatile and comparatively non-volatile organic compounds andhydrates, resulted in composite beads, rods and three dimensionalarticles having superior strength to weight (specific strength),toughness and stiffness compared to pre-pyrolyzed coal dust andsynthetic carbon particles. Thus, the term coal dust is defined hereinto mean any source of carbon dust that performs substantially the sameas raw coal dust, other carbon sources modified with additives thatsimulate raw coal dust, and coal dust that has been pre-heated to lessthan the temperature that would drive off those less volatile organiccompounds and hydrates that cause coal dust to perform as raw coal dustor any coal dust that has been subsequently treated to return suchorganic compounds and hydrates to the coal dust.

In one example, the coal dust used was pure Wyoming coal powder. Whenpyrolyzed at 1000 degrees C. in nitrogen, this coal dust came out as afine, loose powder with a char yield of 57.27%. In another example thecoal dust was Austin Black coal powder. When pyrolyzed at 1000 degreesC. in nitrogen, this coal dust came out as a semi fused gravely mixturethat had to be ground down in order to rewet and make into ceramicsamples, and the fusing of the dust particles was observed to be quitestrong. Coal sticks made with Wyoming coal dust and a polymer derivedceramic material (70/30) had a char yield of about 68.3% whereas thesame process using Austin Black coal dust had a char yield of about 80%.Herein, “about” refers to the mean within industry acceptable variancefrom the mean, which is within plus or minus 20% for char yield of thepure Wyoming coal dust. Linear shrinkage of square rods made withWyoming coal powder and a PDC was about 19.4%. Austin Black sticks madeusing the same process only shrink about 18%. Herein, about means alinear shrinkage of plus or minus 3%. For example, green body 70/30sticks made with Wyoming coal dust and PDC had a density of around 1.13grams per cubic centimeter, which increased to around 1.46 grams percubic centimeter after pyrolyzation. Green body sticks made using AustinBlack coal dust had a density of around 1.2 grams per cubic centimeter,which increased to about 1.7 grams per cubic centimeter afterpyrolyzation. Herein, about means plus or minus 0.2 grams per cubiccentimeter when referring to density. Fracture strength of Wyoming coaldust and PDC composite sticks was about 35 MPa, and Austin Black coaldust sticks using the same process had a fracture strength greater than100 MPa. In one example, the fracture strength was 125 MPa for AustinBlack coal dust composite sticks. Wyoming coal is a lower grade of coaland is considered sub-bituminous. Lower grades of coal usually have morevolatiles than higher grades, as evidenced by the lower char yield andhigher shrinkage. Wyoming coal dust seemed more absorbent to PDC resinsand required longer times or higher temperatures to remove even volatilesolvents.

When comparing Wyoming coal-PDC composites to Austin Black-PDCcomposites, the Austin Black-PDC composites shows no microcracking ormicro-cracking in only a single plane, but microcracking in Wyomingcoal-PDC composite sticks shows some microcracking in both thecompression plane and the lateral plane. This might result from packingthat is excessively tight, causing a rebound effect when pressure wasreleased. In one example, Wyoming coal dust was pre-heated to atemperature high enough to assist in driving off some of the lowertemperature volatile organic compounds and water, which increases charyield and density of ceramic composites made using Wyoming coal dust.Thus, the Wyoming coal dust is modified to perform more like Austin Backcoal dust, resulting in better composite properties followingpyrolyzation. In one example, thermal analysis, such asthermogravimetric analysis (TGA), may be used to compare Austin Blackcoal dust to Wyoming coal dust. The temperature and time to modify theWyoming coal dust may be selected, based on the results of thermalanalysis to define a temperature and time profile for modifying theWyoming coal to be more like Austin Black coal dust. In one example,Wyoming coal powder was dried for thirty minutes at 120 degrees C. toremove moisture, prior to rewetting with a solvent. The mass loss fromthe drying was 3.32% compared to only 1.5% for Austin Black coal dusttreated at the same temperature and for the same time. In comparison,mass loss was 8.5% for Amazon coal dust processed at 120 degrees C. for30 minutes. Thermal analysis may be used to optimize the preheating stepto optimize the properties of coal dust prior to adding coal dust to apolymer derived ceramic or other binder. At this point Austin Black coaldust preheated at 120 degrees for 30 minutes has performed the best incomposite particles and articles. Surprisingly, synthetic graphiteperformed the worst and created composites with macro cracking and earlyfailure in trials. This is a very surprising and unexpected result, asthe nature of the reactions occurring during pyrolyzation of coal dustand polymer precursors to ceramic articles has never been determined orconsidered as an advantage.

In one example, coal dust was pre-pyrolyzed to 1000 degrees C. innitrogen then mixed with polymer derived ceramic resin in a 77 wt % coaldust (Austin Black) to 23 wt % resin ratio, which produces a finishedproduct of the same ratio as the 70 wt % Austin Black coal dust to 20 wt% polymer derived ceramic, without pre-pyrolyzation. 12.06 grams of thepre-pyrolyzed mixture was placed in a 38 millimeter diameter die pressand 14,000 lbs of force was applied. The consolidated disk was removedfrom the die and placed in an oven at 120 degrees C. for 60 minutes tocure. The cured disk had a density of 1.38 grams per cubic centimeter,compared to 1.2 grams per cubic centimeter for a disk formed ofpre-heated but not pre-pyrolyzed Austin Black. The pre-pyrolyzed coaldust has a density similar to pure carbon at 1.6 to 1.8 grams per cubiccentimeter, while Austin Black coal dust has a density of 1.3 to 1.4grams per cubic centimeter after being dried for 30 minutes at 120degrees C. After pyrolyzing the disks in a furnace under nitrogen (asubstantially non-oxidizing atmosphere) at 1000 degrees C., usingidentical firing schedules, the surface of the pre-pyrolyzed samples hadmultiple, randomly oriented cracks, and no cracks were seen in samplesthat were not pre-pyrolyzed. Mass of the pre-pyrolyzed sample was 11.26grams which is 93.36% of the original mass, primarily from the PDCresin. Samples made with coal powder that is not pre-pyrolyzed retainedabout 80% of the original mass. An expected residual mass of PDC isabout 80% of the original. With 23% of a composite formed of PDC, 18.4%of resulting composite would be expected to be contributed by the PDC.Any missing mass probably comes from portions that crumbled off of theedges. It was observed that the sample only shrank 1.5%, which is muchless than the 18% shrinkage of coal dust sticks made withnon-pre-pyrolyzed coal dust composites. The density of pre-pyrolyzedcoal dust ceramic samples decreased from 1.38 to 1.35 grams per cubiccentimeter during pyrolyzation of the composite samples. This is verydifferent than the results for coal dust (not subject topre-pyrolyzation) that increases in density from 1.2 to 1.7 grams percubic centimeter during pyrolyzation.

Sphericity is a measure of how close to a perfect sphere a particle is.Roundness is a measure of how round a projection of a particle is. Thesimple, classical method of quantifying sphericity and roundness is touse a standard sphericity and roundness chart, such as the prior artchart shown in FIG. 1 . Preferably, a proppant has a sphericity of 0.6or greater and a roundness of 0.6 or greater. A sphericity and/orroundness of 0.9 is very difficult to obtain for affordable proppants.To obtain this level of sphericity and roundness, the highest level,some sort of an applied coating or milling is typically required, whichadds expense and may introduce other issues. Generally, a more sphericalproppant has a higher crush strength and fewer fines produced during acrush strength test. The crush strength needed for a proppant generallyincreases as particle size decreases; however, industry prefers largerparticle sizes for formations with lower closure stresses, due to anincrease in the permeability around the larger particle sizes. An idealproppant would have a crush strength, without producing fines,sufficient to withstand expected fracture closure stresses, whileproviding maximum recovery of hydrocarbons. However, no ideal proppantexists and all proppants have trade-offs. Nevertheless, the industry isconstantly looking for an ideal proppant that can be tailored tospecific applications at a low cost. No such proppant exists at a costthat is affordable and meets or exceeds all of the properties preferredby industry. The processes described in U.S. Pat. No. 8,961,840, whichare incorporated herein by reference in their entirety, are provided asexamples for processing the slurry, wherein coal dust may be added in arange greater than 40% but not greater than 90% by weight of coal dustto polymer derived ceramic precursor, more preferably at least 60% andnot greater than 80%. As needed, a solvent or nonsolvent, such as afugitive nonsolvent, may be added to reduce the viscosity of the slurryduring processing, for example. The application for these types ofmaterials is limited to those niche applications where cost is veryimportant, because other fillers are available that perform better thancoal particles, such as carbon black and fly ash, for example.

FIG. 4 shows an example of the most important calorific constituents ofcoal in several different forms. For example, it is known in the art howthese constituents change from the as received (as), air dried (ad), drybasis (db) and dry ash free (daf) conditions. FIG. 5 schematically showsan example of a coal particle with included minerals, excluded minerals,an organic coal matrix and organically associated elements. FIG. 6 showsan example of a chemical symbol for one type of coal, which shows thatcoal can be a complex molecular structure. FIGS. 7(a)-(b) show twoexamples for chemical equations for coal, showing the difference betweenbituminous coal and anthracite (a type of coal that has undergoneadditional pressure and heat in formation of mountains and the like).FIG. 7(c)(1-4) divide coal into 4 major ranks and show images of threeof the four ranks. FIG. 8 shows the change in hydrogen, carbon andoxygen in some types of coal.

FIGS. 9A-C shows three different types of commercial equipment formaking particles. FIG. 9A is disk pelletizer. FIG. 9B is pin mixer, andFIG. 9C is a granulator.

SUMMARY

A proppant is comprised of a complex composite particle, such as apyrolyzed mixture of coal dust and polymer derived ceramic composite.For example, no more than 90 percent coal dust but no less than 40percent coal dust is mixed with a polymer or polymers, comprising theremainder of the mixture (i.e. 10 percent to 60 percent). One or moresolvents and other additives, such as surfactants, binders,antimicrobials, antifungals and stabilizers may be added to the mixtureto create a slurry. Then, the slurry may be further processed to produceceramic beads comprising a plurality of ceramic constituents derivedfrom the coal dust and polymer. In one example, the particles of coaldust are prepared from coal, such as by pulverizing, crushing and/ormilling of coal into a fine dust. Then, the fine dust may be sorted bycentrifuging, screening or filtering, such passing the dust throughsuccessive sieves, to be within a pre-selected, uniform particle sizerange (PSR). In one example, a single PSR is selected. In anotherexample, coal dust is selected having a plurality of PSR's, such as acertain fraction having a first PSR and another fraction having adifferent PSR. The mixing of two or more different PSR's may be selectedto create various microstructures within fired and processed beads madefrom the mixture of PSR's and a polymer derived ceramic matrix.

Alternatively, a proppant may be made from beads of coal dust and amatrix other than a pyrolyzed ceramic. For example, a complex compositeparticle may comprise a pyrolyzed mixture of coal dust and polymerderived ceramic beads and a polymer matrix and/or shell. For example, asingle bead may be covered by a wax or polymer shell. Alternatively, aplurality of beads may be bound together by a polymer matrix and/orshell in a honeycomb array, for example.

In one example, coal dust evolves volatile organics and water during ticfiring of beads formed from the slurry, chemically changing thecomposition of the coal dust and increasing the density of complexparticles formed. Alternatively, porosity may be added further reducingthe density of the beads by adding porosity to the ceramic beads. Theporosity may be closed cell or open cell pores within the ceramic beads.If open celled porosity, the open cells may form a continuous network ofporosity only within the coal dust particles or may extend throughoutthe ceramic beads. The amount of polymer may be varied, such that thepolymer serves either as a matrix with coal dust filler or as a binderfilling the spaces between compacted coal dust particles. In oneexample, where the polymer derived ceramic is formed from a polymer thatserves as a binder for coal dust particles, the percentage of coal dustto binder may be from 60% by weight of coal dust to total weight of thecoal dust and binder to 90% by weight of coal dust to total weight ofthe coal dust and polymer derived ceramic binder, more preferably 65% to75%. The properties of the polymer derived ceramic binder, the interfacebetween the polymer derived ceramic binder and the coal dust, and theindustrial requirements of the proppant determine the preferredpercentage of coal dust to total weight of the coal dust and binder. Inone example, an external layer of the ceramic bead is comprised of alayer of the pyrolyzed polymer derived ceramic, substantially free ofcoal dust particles, which may be slip layer, for example. For example,the compressive strength of a bead of pyrolyzed 100% coal dust would benegligible; however, the fine coal dust has a much higher compressivestress, when formed into a composite with a polymer derived composite.Preferably, the polymer derived composite is one that produces a beadwith a sphericity and roundness greater than 0.9 and a compressivefracture strength of at least 2,000 psi, more preferably 5,000 psi, evenmore preferably 10,000 psi. Preferably, the coal dust is incorporatedinto the composite bead in a way that does not unnecessarily underminethe compressive strength of the matrix or binder. In one example, a coaldust-ceramic composite meets all of the requirements of the petroleumindustry for a proppant at a cost less than 50 cents per pound, morepreferably less than 25 cents per pound.

“tic” is an adjective referring to “sis,” the heating, ordinarily tovery high temperatures, such as 400 to 1000 degrees centigrade, oforganic materials, such as polymers, with or without other constituentsand additives, to form a ceramic. Such tic heating results in anirreversible, thermochemical decomposition of the organic materialsand/or additives at the elevated temperatures, in the absence of oxygenand/or any halogen, resulting in simultaneous changes in chemicalcomposition and physical phase. tic firing may be completed for greenbodies and/or beads. In one example, green bodies are heated and atleast partially dried and are then pyrolyzed. Then, the ceramic may befurther mechanically and/or chemically processed to form ceramiccomposite beads of a preferred size and shape. For example, nearlyspherical beads may be formed by milling and or processing. In oneexample, surface tension effects are used to drive the formation ofbeads having a sphericity of no less than 0.9 and a roundness no lessthan 0.9. In one example, substantially no alumina is present in theceramic beads.

For example, FIG. 10 shows some standard coal sizes, and smaller sizesmay be obtained by pulverizing and/or milling coal into finer and finerparticle sizes. Particle sizes of the beads may be tailored to anypractical size required by the industry. Particle sizes (PD50) may beprepared from 10 microns to 1000 microns, depending on the fractureclosure stresses specified, which is an extraordinarily surprising rangeof particle size for the composite beads. More preferably, a range from20 microns to 500 microns meets or exceeds all of the industry standardsfor closure stresses up to 2,000 psi. Even more preferably, beads withmedian particle sizes (pd50) from 50 microns to 200 microns meet orexceed all of the industry standards up to a fracture closure stress of5,000 psi. It is thought, without being limiting in any way, that coaldust filler with small particle sizes, when mixed together with polymerderived ceramic materials, provides the benefits achieved by smalldiameter proppants, while the polymer derived ceramic, with its greaterstrength and sphericity provides the advantage of larger particle size.Thus, composite spherical beads with dispersed coal dust within aceramic matrix or binder may be tailored to achieve whatever propertiesare needed for a proppant by the petroleum industry. Surprisingly, theporosity introduced by the coal dust and fugitive solvents andnonsolvents are less of a problem at percent additions of coal dust tothe combination of polymer and coal dust greater than 40 percent byweight of coal dust to total weight of coal dust plus polymer used forthe polymer derived ceramic matrix or binder. These two mainconstituents, not considering the mass of fugitive binders, solvents andnonsolvents, provide the constituents of the beads after pyrolyzation.In one example, a ceramic bead resistant to crushing at a specificfracture closure stress is desired, in addition to low cost and adensity less than 1.5 g/cc. In this example, a ceramic bead may betailored for the specific conditions desired by adjusting the size ofcoal dust particles or PSR's, the percentage of coal dust, the presenceand thickness of a slip layer on the surface of the bead and the beaddiameter selected for use as a proppant in a fracking fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings are illustrative examples and do not furtherlimit any claims that may eventually issue.

FIG. 1 discloses a prior art graph for determining sphericity androundness, according to a standard for fracking proppants, such asfracking sand.

FIG. 2 is a graph showing data for fracture stress resistance forvarious coal dust compositions in coal dust-polymer derived ceramiccomposite beads.

FIG. 3 illustrates a cross section of an example having a core and acoating, wherein the core comprises coal dust particles and the coatingis substantially free of coal dust particles.

FIG. 4 illustrates a prior art calorific break down of coal.

FIG. 5 schematically illustrates a prior art conception of coalparticles.

FIG. 6 illustrates an example of a chemical symbol for one type andsource of coal in the prior art.

FIG. 7 (a)-(c 4) illustrate chemical equations for bituminous coal andanthracite, and show images of three of the four major categories inwhich coal is classified in the prior art.

FIG. 8 illustrates a prior art comparison of different coal broken outby different recognized types of coal, showing a significant variationin hydrogen, oxygen and carbon.

FIGS. 9A-C illustrate machines for processing wet powders to formproppants.

FIG. 10 illustrates a prior art example of standard coal sizes used inthe coal.

FIG. 11 shows an example of an emulsifier process.

FIG. 12 shows an example of a spray drying process.

FIG. 13 shows an example of a coal-binder “clay” extrusion process.

FIG. 14 shows a graph of flexural strength in MPa versus density in g/ccof composite test bars made of coal-polymer derived ceramic precursorbeads after pyrolyzation of the test bars.

FIGS. 15(a)-(b) show (a) results for testing of a coal dust-polymerderived ceramic test bar and (b) an image of a resulting amorphousphase.

FIG. 16 schematically illustrates a cross section of a coal dust-binderbead with the binder shown by cross hatching.

FIG. 17 schematically illustrates a cross section of a pyrolyzedcoal-dust-polymer derived ceramic bead with dashed lines showing anamorphous phase.

FIG. 18 schematically illustrates a complex ceramic particle formed of aplurality of coal dust-binder beads with a matrix.

FIG. 19 schematically illustrates a complex ceramic particle compositeformed from a plurality of complex ceramic particles within a matrix.

FIG. 20 schematically illustrates a structure comprising the complexceramic particle composite incorporated into the structure.

FIG. 21 shows a graph of a thermogravimetric analysis.

FIG. 22 shows a graph of another thermogravimetric analysis.

When the same reference characters are used, these labels refer tosimilar parts in the examples illustrated in the drawings.

DETAILED DESCRIPTION

In one example, more than one-half of the composite, by weight, iscomprised of coal dust, and the particle size (pd50) of the coal dustparticles is at least 10 times smaller than the composite bead particlesize, more preferably 20 times smaller, even more preferably 50 timessmaller, yet more preferably 100 times smaller. For example, the medianparticle size (pd50) of the coal dust may be selected in a range from 10to 200 microns, and median bead size may be selected in a range from 100to 2000 microns. The coal particles may be agglomerated using a resinbinder to form beads. A polymer derived ceramic resin may be used toform a coal-ceramic composite bead.

In one example, the resin binder for the coal dust is a polymer derivedceramic precursor. In another, the binder is a fugitive binder. In yetanother, the binder helps to functionalize the surface of the coal dustparticles.

For example, when a resin to coal particle ratio is greater than 1:1,then beads of resin/coal may be formed by emulsion or spray drying. Forexample, FIG. 11 shows mixture of resin/coal dust being fed into anemulsion tank 1102. Optionally, a mixer or sonicator prevent theemulsified resin/coal drops from coalescing. Heat 1103 may be added tocure or partially cure the beads that are extracted and captured in aseparator 1105. Alternatively, beads may be formed by spray drying, asschematically shown in FIG. 12 , for example. The slurry is fed througha spray nozzle 1101′ and separates into droplets that may be cured orpartially cured by heating 1103 and are collected in a separator 1105.For example, a resin to coal ratio from 40:60 to 50:50 may be processedas a moldable clay, as illustrated in the example of FIG. 13 , forexample. An extruder ram 1304 may force the “clay-like” mixture througha rotating screen 1302. Then, optionally, an hydraulic clay ejector mayuse a fluid to eject the beads 1303 from the screen 1302 into aseparator 1306, for example. For mixtures of binder to coal dust thatare less than 40:60, then the mixture may take on the characteristics ofa damp powder. Damp powders may be processed by one of the machines inFIGS. 9A-C, for example, such as (A) a disk pelletizer, (B) a pin mixeror (C) a granulator.

The resulting beads may be further processed to add a surface coating,which may be the same material as the binder or may be a differentmaterial. The surface coating, if any, may be processed by spraying,rinsing, adding the coating resin to the emulsifier process orotherwise. The coating may be post-processed by heating or curing orpartially curing. For example, if the beads are going to be used in acomposite, then a tacky surface coating may be desired to aid inagglomeration and/or sticking of one bead to another. For example, beadsmay be used in three-dimensional printing of composite articles.

The beads may be spheroidized and pyrolyzed to form a compressionresistant proppant. A resulting fracture closure resistance of a ceramicbead comprised of a composite of the coal dust and a polymer derivedceramic may be selected in a range from 2,000 psi to 20,000 psi. In oneexample, a composite ceramic bead is formed with a coal dust and ceramiccore and a ceramic shell, substantially devoid of coal dust, and theceramic bead may have a resistance to fracture stresses between 6,000psi and 20,000 psi. When pyrolyzed, the ceramic shell binds to theceramic portion of the core and forms a hardened, nonporous shell, andthe core comprises coal dust, porosity and a cancellous ceramic networkwithin a compact shell, mimicking the structure of bone, for example.Alternatively, the shell may be selected as a wax or polymer shell,instead of a ceramic shell. Any suitable wax or polymer may be selected.

The core of a ceramic precursor bead may be processed and at leastpartially dried and heated, such as at a temperature up to 400 degreescentigrade in a spray drying chamber. Then, the core may be furtherprocessed by mixing with a polymer, and the core may be coated with aslip layer of a polymer. Then, the slip layer of the polymer, which maybe referred to as a “slip layer,” herein, may produce a polymer derivedceramic, when the core and slip layer are pyrolyzed. The slip layer maybe the same polymer derived ceramic as the polymer derived ceramic phasein the composite core. Alternatively, a different polymer derivedceramic may be selected for the slip layer. The thickness of the shellmay be tailored by controlling the viscosity of the polymer when mixedwith a plurality of the cores and by the choice of processing stepsfollowing mixing. Slip layers may be added and dried repeatedly in anautomated process that provides a layered shell or by a coating with anatomized layer of polymer during coating of a fluidized collection ofbeads suspended within a fluid stream, for example.

Collectively, a plurality of slip layers or a built up slip layer, overtime, may form a single ceramic shell when pyrolyzed or a plurality ofceramic shells may be tailored for particular attributes, such asresistance to fracture closure stresses and/or hardness and/or toughnessand/or fatigue resistance. In one example, the shell forms from aplurality of a layers. For example, a tough layer, such as a plastic,crack blunting material may be interspersed with a hard layer, such as aceramic material. The layering may mimic natural protective shells, suchas those of marine life, for example.

For example, the cores may be separated into individual beads, such asby spray drying or emulsion forming or any other process capable offorming bead-like core particles, prior to pyrolyzing the beads at apyrolyzation temperature, such as 400 degrees centigrade or greater,more preferably at a temperature selected in a range from 600 degreescentigrade to 1000 degrees centigrade, forming a solid or nearly solidouter shell on a composite core comprising coal dust/polymer derivedceramic, wherein the core may contain substantial porosity, withoutunnecessarily adversely affecting the crush strength or fracturecompression stress resistance of the ceramic composite proppant.

A fraction or percentage of coal dust to coal dust and polymer, byweight, may be selected for the composite core. For example, a fractionof coal dust may be selected in a range from about nine-tenths to aboutsix-tenths of the total coal dust and polymer, by weight, morepreferably the percentage of coal dust to combined coal dust and polymeris selected to be from 80% to 60% coal dust by weight, within the core,prior to heating the core, resulting in a significant reduction indensity and cost of the core material. The shell may comprise a thinceramic film, such as a 0.1 micron film or a thicker coating may beapplied to the core, greater than 0.1 microns. In one example, athickness of the slip layer coating of a pyrolyzed ceramic bead isselected from 1% to 20% of the particle's effective diameter, even morepreferably from 5% to 15% of the particles' effective sphericaldiameters. The thickness of the ceramic coating may be tailored as atrade-off between cost and fracture closure strength resistance of thecomposite ceramic beads, for example.

In one example, the ceramic beads comprise porosity, further reducingthe density below the density of a mixture of the carbon and binder orpolymer derived ceramic materials. It is believed, without beinglimiting in any way, that increasing porosity will decrease thecompressive strength of the ceramic beads. However, porosity at the coreof a ceramic bead has less effect on the crush strength of a ceramiccomposite bead than porosity at the outer shell. Thus, coal dust-polymerderived ceramic beads may be tailored having a wide range of resistanceto crushing by fracture closure stresses, cost, sphericity/roundness anddensity. These parameters may be selected by processing steps, ratios ofbinder to coal dust for the core, and selection of the type of coal dustand particle size. It is thought, without be limiting in any way, thatchemical processors occur during pyrolyzation of the core, resulting inoutgassing that can form porosity.

For example, a roundness or sphericity of the ceramic beads may be atleast 0.9 after processing of the beads. A cost of the beads may be lessthan one-half U.S. dollar per pound, more preferably less than U.S.$0.25 cents per pound. For example, the ceramic beads, when added to aproppant, are capable of withstanding closure pressures of at least 6000pounds per square inch (psi), more preferably at least 7000 psi, evenmore preferably at least 8000 psi, yet more preferably at least 10,000psi. In one example, the specific gravity (compared to water) is nogreater than 2.6, more preferable no greater than 2.5, even morepreferably no greater than 1.5, still more preferably about 1.0, suchthat the ceramic beads remain suspended in and carried along with aproppant fluid having a density similar to water, and at a concentrationsuch that the viscosity of the fracking fluid is not unreasonablyelevated by the addition of the proppant.

For example, composites made from consolidating a variety of beads andpyrolyzing the composite demonstrates a flexural strength as shown inFIG. 14 . The graph shows flexural strength in MPa versus density ing/cc of composite test bars made of coal-polymer derived ceramicprecursor beads after pyrolyzation of the test bars. Pressing the greenbody at higher consolidation pressures increases flexural strength anddensity. FIG. 15(a) shows some properties of an example of a 100 mm×7mm×7 mm test bar made of coal-polymer derived ceramic beads. The resultsare surprising and unexpected, as is the glassy microstructure shown inFIG. 15(b) in which some porosity is evident. It is thought, withoutbeing limiting in any way, that the coal particles chemically react andbind with the ceramic matrix binder forming an amorphous material.Surprisingly, without being limiting in any way, the process may producefullerenes, nanotubes and/or nanofibers in situ during the pyrolyzationstep. It is thought, without being limiting in any way, that iron orpreexisting nanostructures may be important in the growth of thesestructures during pyrolyzation. It is known that there is hydrogenavailable in the sub-bituminous and bituminous coal used in some ofthese examples, which could produce an environment, locally, suitablefor fullerene, nanotube and/or nanofiber growth.

For example, a polymer derived ceramic may comprise a polysiloxane,polysilazane, polyborosiloxane, polycarbosiloxane, polyborosilane,polycarbosilanes and combinations of any two or more of these, with orwithout other additives. In one example, a precursor for the polymerderived ceramic is mixed with a low-cost filler, such as coal dust. Inone example, a critical range for the low-cost filler, such as coaldust, comprises a fraction of coal dust to polymer derived ceramic in arange from no greater than 9 parts coal dust to 1 part polymer derivedceramic to no less than 6.5 parts coal dust to 3.5 parts polymer derivedceramic, by mass. Preferably, the fraction of coal dust is greater than6 parts coal dust to 4 parts polymer derived ceramic. Surprisingly, acore having greater than 6 parts coal dust to 4 parts polymer may have agreater fracture resistance than a core with less coal dust, as shown inFIG. 2 , for example.

For example, a carbon-based, organic material, such as a source of coal,may be pulverized into a dust. The coal dust may be separated intodifferent particle sizes, such as by passing the coal dust through asieve or sieves. Sequentially sieving the coal dust through a series ofsieves gives an indication of the coal dust particle size, For example,a No. 270 sieve has openings of 53 microns, a No. 325 sieve has openingsof 44 microns, and a No. 400 sieve has openings of 37 microns.Therefore, each successive sieve traps particles larger than the openingsize through the sieve. If the particles are not spherical, then thesieve opening usually reflects the second maximum dimension of theparticles, i.e. not the length but the width of a particle. For example,coal dust particles having an equivalent diameter of 40 microns mightpass through a No. 400 sieve, if non-spherically elongated in onedirection and less than 37 microns in its other dimensions. Also, asieve may be used for screening out larger particles or agglomerationsof particles prior to mixing with a polymer. Coal dust that passesthrough a 37 micron sieve may be utilized in composite beads orcomposite bead cores having equivalent diameters from 150 microns andgreater, more preferably 200 microns to 2000 microns, even morepreferably 400 microns to 800 microns, for example. Ultracentrifuges andother techniques may be used to separate out particles smaller thansieves with the smallest available holes, for example, and othertechniques may be used to characterize particles, even down to a fewmicrons. Synthetic forms of carbon may be utilized (usually at a highercost) if available. For example, graphite powders may be availablealready sifted to very small particle sizes. The type of carbon powderutilized may affect the porosity and density of the core and may have anadverse or beneficial impact on the cost, fracture stress resistance,sphericity/roundness and density of carbon-polymer derived ceramicbeads.

In one example, a coal dust may be mixed with a polymer, such as apolymer to ceramic material. For example, a polymer to ceramic materialmay be selected from Starfire Systems, such as an SPR212, which forms asilica carbide ceramic upon heating in a tic process. Alternatively, apolymer may selected to form other ceramic materials including a metalor rare earth and oxygen, carbon, nitrogen, boron or combinations ofoxygen, carbon, nitrogen and boron, for example. For example silicon mayform a ceramic with carbon (SiC) or with oxygen (SiO) or with carbon andoxygen (SiOC). Ceramics may be comprised of oxides, carbides, borides,nitrides and the like. The resulting ceramic and the properties of theceramic will depend, significantly, on the processing used to producethe ceramic or ceramic composite beads.

In one example, a nonsolvent, such as water or other non-dissolvingfluid, is added to form a slurry of the coal dust and polymer, and thepolymer may be dissolved in a solvent or may be present as polymerparticles or as a liquid polymer at the temperature of the beadformation. For example, a slurry may be formed into small droplets,which partially cure to form beads, such as by drying and/or heating thebeads as the beads are formed. For example, beads with mean particlesizes selected in a range from 100 microns to 2 millimeters may beformed having a sphericity of 0.9 or greater. The partially cured beadsmay be fully solid or may have porosity from volatilization of asolvent, a reaction product or a nonsolvent. In one example, the amountof solvent or nonsolvent is selected to control the amount of porosityin the bead, tailoring the density and crush strength of the bead. Inone example, the polymer may be dissolved or partially dissolved in asolvent, and the polymer solvent may be a nonsolvent to the carbon dust.Alternatively, an additional nonsolvent may be added to reduce theviscosity of the mixture of solvated polymer and coal dust. For example,ceramic beads may be formed having bead sizes from 30 to 100 microns, ifcoal dust size is selected to be no greater than 3 microns. Morepreferably, median bead particle size (pd50) is selected to be in arange of 50 to 2000 microns, depending on the specifications required bythe fracking industry. In one example, a 30 micron bead is formed bymixing very fine coal dust, 3 microns or smaller, with a polymer usedfor creating a polymer derived ceramic and a solvent. The mixture issprayed through a nozzle into a spray dryer forming nearly sphericalbeads. The beads are pyrolyzed, such as at a sis temperature of 1000degrees centigrade and in a non-oxidizing atmosphere, wherein anirreversible chemical reaction occurs. Hydrates, water vapor andvolatile organic compounds are volatilized from the coal dust andpolymer of the bead during hearting, drying and sis, resulting in aporosity in the core of the bead, reducing the density of the bead toless than 1.5 g/cc, in one example, more preferably less than 1.2 g/cc.In one example, the mean sphericity is no less than 0.9 and the meanroundness is no less than 0.9. The sphericity of coal dust is notparticularly important to the sphericity of a composite bead made from aplurality of coal dust particles and a polymer derived ceramic.Preferably, each ceramic bead comprises at least 10 coal dust particlesand has a sphericity and roundness no less than 0.9.

In one example, a process includes a spray dryer. A spray drying processmay comprise a nozzle, such as a sonic, acoustic and/or electrostaticnozzle. The orifice and type of the nozzle may be selected to outputparticles of a defined size range and sphericity/roundness. In somenozzles, mixing may occur at or immediately adjacent to the nozzle,allowing the coal dust to be fed into one inlet and the polymer at asecond inlet. The composite beads are emitted from the nozzle uponmixing of the two feed stocks. For example, the temperature andatmosphere in a drying chamber may be controlled to cure the beads,partially or completely, by conventional heating elements orelectromagnetic waves, for example. Waves of any frequency may beprovided to impart heat or to directly cure the polymer, such asinfrared waves, microwaves, ultraviolet light, x-rays or the like. Theresident time in the drying chamber may be shorter or longer dependingon the rate and degree of curing and drying desired. sis may occur inthe drying chamber or may be completed after the beads are removed fromthe chamber or in a subsequent chamber, in a continuous or batchprocess. Alternatively, the beads may be defined as having a coredistinct from the outer layer or layers of the beads, by furtherprocessing. For example the core may be mixed with a polymer, such as apolymer derived ceramic polymer, which may be the same or different thanthe polymer derived ceramic polymer of the core. The core may bepyrolyzed (i) to completion, (ii) for the first time or (iii) for asecond time, after being dried and coated with the polymer derivedceramic layer. In one example, the core is partially dried, and would betacky to the touch, when a slip layer of a polymer for an externalpolymer derived ceramic coating is applied to the core. Alternatively,after being coated, the bead may be processed through a nozzle and/or adrying chamber and/or an emulsion to separate the beads and cure thecoating, prior to or at the same time as the beads are being pyrolyzed.

FIG. 2 shows a fracture stress comparison for bead cores for fourdifferent percentages of coal dust in the core. The 0% coal dust is dataderived from polymer derived ceramic beads, and the error bar shows arange of fracture stress from low to high based on choice of polymer andprocessing parameters. The fracture stress of the 70% coal dust-polymerderived ceramic was especially surprising and unexpected, almostachieving a 10,000 psi fracture stress for a core at. The density ofpolymer derived ceramic is about 2 glee. The density of coal dust isabout 0.6 g/cc, depending on the type of coal, and bulk density of coalmay be as high as 0.93 glee. However, a pyrolyzed mixture of coal andpolymer derived ceramic my have a substantially different density thanthe constituents, due to volatile gases and processing differences. Thedata points in FIG. 2 are for beads having a density of 2.0 to 2.1 glee(0% coal dust) and a bulk density of 1.27 g/cc (70% coal dust) wasmeasured from a rod formed of compacted beads that was then pyrolyzed,by measuring the mass and volume of the rod; and a bead made from onlycoal dust is assumed to have a density less than 0.93 g/cc (highest bulkdensity for coal) and probably much less. The density of the beads with40% coal dust, which were made by emulsion processing, has not beenmeasured, yet. The density is probably less than 1.27 g/cc, but thedensity difference cannot account for the significant difference incomparative fracture stress resistance. Ordinarily, one would predictthat both the density and comparative fracture stress would be inverselyproportional to the percentage of coal dust; however, the data shows asubstantial variation in comparative fracture stress with the choice ofpolymer precursor, percentage coal and processing parameters for a beadcore that defies conventional assumptions. This provides opportunitiesfor tailoring properties of proppant beads made of a composite of coaldust and polymer derived ceramic, because the data for 70% coal dust hasa very favorable density, fracture stress resistance, sphericity,roundness and cost that make it attractive as a proppant, meeting orexceeding industry requirements.

In one example, the 70% coal dust-30% polymer derived ceramic compositebead was formed by mixing together 7 parts by weight of coal dust(approximate particle size of 3-5 microns) with 3 parts by weight of apolymer derived ceramic precursor, such as an SPR212 or other polymercapable of being pyrolyzed to form a ceramic. Additionally, chloroform,a solvent for the polymer, was added at 1 part chloroform to 1 part ofthe mixture by volume, producing a slurry. The slurry was emulsified toproduce beads that were partially cured at a temperature of 50 degreesC. for 30 minutes. The beads were transferred to a retort furnace underan atmosphere of dry nitrogen for 600 minutes at room temperature andramped to 1000 degrees centigrade over ten hours, causing sis of thepolymer to form a ceramic. The composite bead particle size wasestimated to be 400 microns. Without being limiting in any way, it isbelieved that the polymer derived ceramic acted as a binder for the coaldust particles. For example, the composite ceramic beads in the exampledisclosed in FIG. 2 are likely to meet or exceed industry requirementsfor a proppant.

In another example, a 40% coal dust-60% polymer derived ceramiccomposite bead was formed by mixing 4 parts by weight of coal dust(approximate particle size of 3-5 microns) with 6 parts by weight of apolymer derived ceramic precursor, such as an SPR212 or other polymercapable of being pyrolyzed to form a ceramic. Additionally, chloroform,a solvent for the polymer, was added at 1 part to 2 parts of the mixtureby (weight/volume), producing a slurry. The slurry was emulsifiedproduce beads that were (dried/partially cured) at a temperature of 50degrees C. for 30 minutes. The beads were transferred to a retortfurnace under an atmosphere of dry nitrogen for 600 minutes at roomtemperature and ramped to 1000 degrees centigrade over ten hours,causing sis of the polymer to form a ceramic. The composite beadparticle size was estimated to be 400 microns. Without being limiting inany way, it is believed that the polymer to ceramic material comprised amatrix, and the coal dust may have introduced defects and voids into thematrix, such as by outgassing from the coal dust and evaporation of(solvent/nonsolvent) during sis. In the example show in FIG. 2 , theresulting composite beads failed to meet industry requirements for aproppant.

Before conducting the experiments, one would have assumed that anyprocess and any percentage of coal dust less than 90% coal dust couldhave at least met the industry standards for a proppant, but the resultsof experiments have shown that less coal dust does not necessarilyresult in a higher comparative fracture stress, which is one of the mostcritical requirements that any fracking proppant must meet.

More generally, proppants may be made by a variety of processes,provided that the polymer to ceramic phase acts, preferably, as a binderbetween coal dust particles, and with a range of coal dust percentages,provided that the coal dust particles and any porosity introduced duringprocessing do not become defect sites undermining the composite bead'sresistance to fracture stress. Whether or not such defects exist willdepend on processing conditions, coal dust particle size,solvent/nonsolvent characteristics, the fracture stress and toughness ofthe polymer to ceramic phase and whether the evolution of a vapor phaseduring sis adds porosity to the composite beads.

FIG. 3 illustrates, schematically, an example of a composite ceramicbead comprising a plurality of coal dust particles 11, which may besubstantially non-round and non-spherical, a polymer to ceramic phase12, which may act as a binder, a plurality of pores 13, forming closedporosity or interconnected, open celled porosity, within the polymer toceramic phase, and a coating 14, which may be a slip layer,substantially free of coal dust. As shown, the core coal dust-polymer toceramic portion of a bead may be non-spherical and non-round, and theslip layer may increase the sphericity and roundness of the bead. Also,the bead core may have a substantially lower fracture stress than thebead shell, the shell providing the greatest contribution to fracturestress resistance of the bead. Thus, the bead may comprise a coreincluding up to 90% by weight coal dust to polymer derived ceramic,while the composite bead meets or exceeds industry requirements for aproppant. The representation in FIG. 3 shows a coal dust particle thatseparated from the core and found its way into the shell; however, thepresence of an isolated coal dust particle 15 or even several coal dustparticles within the slip layer has little effect on the fracture stressresistance of the composite bead, provided the slip layer issubstantially free of coal dust, meaning that the slip layer comprisesless than 10% by weight of coal dust to polymer derived ceramic and coaldust particles do not extend beyond the surface of the polymer toceramic slip layer. The addition of a slip layer 14 may improve,substantially, the sphericity, roundness and fracture stress resistanceof a composite bead, even if a composite core of the bead would not havemet industry requirements for a proppant.

FIG. 16 illustrates a bead 1600 formed by coal particles 1601, such asillustrated in FIG. 5 , and a binder 1602. The binder:coal particleratio may be any of the ratios disclosed, and the relative proportionsin the drawing are merely an example. If the binder is a polymer derivedceramic precursor, then the cross sectional microstructure afterpyrolyzation may resemble the schematic illustration in FIG. 17 , forexample. FIG. 17 shows a pyrolyzed coal dust-polymer derived ceramicbead 1600′ with an amorphous matrix phase 1610 and inclusions, such asinorganic inclusions 1613, nanotubes/nanofibers 1611, and porosity 1614.The porosity 1614 may be interconnected or closed.

In FIG. 18 , a complex particle is formed of a plurality of coaldust-hinder or coal dust-polymer derived ceramic beads 1801 within amatrix 1802, for example. The matrix 1802 may be any material. In oneexample, the matrix is formed from a coating applied to the beads 1801.For example, the complex particle 1800 may have a slip layer or sliplayers 1803, 1804 applied to its surface. In FIG. 19 , a plurality ofcomplex particles 1800 are consolidated in a composite body. Thecomposite body 1800 may be any composite body and take on any shape withany thickness. In one example, the composite body is formed by threedimensional printing by spraying or depositing the complex particles1800, such as by suspending them in a fugitive carrier fluid. In FIG. 20, the three dimensional shape of the composite body 1900 is part of acomposite structure 20. The composite structure comprises a compositebody core 1900, a first surface layer or layers 2001 on one surface anda second surface layer or layers 2002 on an opposite surface.

The beads 1600, 1600′ may be used to make larger complex particles 1800that may be consolidated and compressed, for example, at a highpressure, to form a complex particle body 1900 of a composite structure2000, for example. The beads 1600, 1600′ may be pre-processed to add asealing layer on the beads before incorporating the beads into a complexparticle 1800, for example. The sealing layer may be any material thatprevents or reduces penetrating of the matrix phase 1802 into the poresin the beads 1600, 1600′, for example. Alternatively, the pores may beintentionally filled by a pore filling material, preferably one thatwets the surfaces of the beads in order to reduce porosity. The porefilling material may be low density and/or a ductile crack arrestor, forexample.

Polymer derived ceramics, PDC's, may be oxide and/or non-oxide PDC's,such as SiC, SiOC, HaC, BC, and the like and/or nanoparticles of metaloxides such as SiO2, Al2O3, and Fe2O3 and the like, and/or nanoparticlesof clays such as Kaolinite, Illite, Calcite or Pyrite and the like,and/or nanofibers and/or nanotubes of carbon, silicon, silicon carbide,silicon oxycarbide and the like, which may be added or grown in situ,and/or nanosheets of graphene and/or nanodomains of graphitic carbon.These constituents may be bonded together chemically by thermal sis toform a nanoporous bead, particle or bulk material, for example. Afterbeing mixed together, or as they are being brought together (threedimensional printer mixing constituents together on the fly), prior tothermal sis, the constituents may be fabricated into complexthree-dimensional parts. For example, three-dimensional printing,extrusion, rolling, molding and other forming processes may be used tocreate three-dimensional green bodies. The fabricated parts would thenbe cured to a rigid green part which may be thermally pyrolyzed toproduce a ceramic nanocomposite part.

Two main sources of the constituents are polymer derived ceramics(PDC's) and coal. PDC's are a type of ceramic that is derived from aliquid or solid polymer precursor which is cross-linked or cured into asolid material which is then pyrolyzed at high temperatures whichconverts the polymer into a ceramic. For example, the ceramic matrix andcoal dust may form an amorphous phase within coal-ceramic particles. Aplurality of coal-ceramic particles may be agglomerated into a complexcomposite particle. Ceramics, such as SiC, SiOC, HaC, BC, may beproduced from polymer precursors. Depending on the desired finalproperties, a powdered coal may be mixed with a liquid PDC resin inamounts that could range from 1 wt % PDC resin and 99 wt % coal all theway to 99 wt % resin and 1 wt % coal. However, for most applications,the range would be much narrower.

Once coal and liquid resin are thoroughly mixed and beads are formed, astructure may be formed from the beads and may be molded, rolled,extruded, three-dimensionally printed, sprayed, dipped or otherwiseprocessed into complex shapes or coatings. The shapes or coatings wouldthen be cured to a rigid material. Pyrolyzation in a furnace, underinert atmosphere to 800 C to 1500 C, depending on the PDC system beingused, can turn the green body into a ceramic nanocomposite. Theresulting ceramic nanocomposites possess high strength, high stiffness,high toughness, and high temperature resistance as well as othersurprising and unexpected properties, such as electrical conductivityand selectable thermal expansion properties, among others. The uniquenanostructure produced by the processing of particles to beads tocomplex particles to three-dimensional structures is enhanced bychemical processes that occur during the sis. At the beginning of thesis transformation in the temperature range of room temperature to 150degrees C., moisture in the coal powder evaporates. This processproduces nano pores within the coal particles and/or coal-binder orcoal-PDC bead. In the temperature range of 400 degrees C. to 600 degreesC., volatile low molecular weight aromatic and aliphatic hydrocarbons,such as benzene and methane evaporate, which further increases nanoporosity. Also in this temperature range, PDC resin starts itstransformation to ceramic. This also involves the evolution of gasessuch as methane, CO and CO2. An important structural transformation alsostarts in this temperature range. There are many different traceelements present in coal, such as nickel, tin, iron and the like. Forexample, the mineral pyrite, FeS2, is in sources of coal tested. Atthese temperatures, pyrite decomposes into elemental iron and sulfurgas. Sulfur gas dissipates leaving elemental iron, which may be presenton the surface of the coal particles and on inner walls of newly formednanopores within beads. Nickle and iron both catalyze formation offullerenes, carbon nanotubes and nanofibers from methane and other lowmolecular weight hydrocarbons within the nanopores. At this point thereare methane and hydrocarbon gases being generated from the decompositionof the coal and PDC resin. These gases may be flowing through thenanopores. As these gases find elemental nickel and iron, in situ growthof fullerenes, graphene, nanotubes and nanofibers, such as of carbon,silicon carbide and like, may spontaneously grow, sometimes as long ashundreds of microns and may attach one to the other or from one particleto another, increasing strength and toughness to a surprising degree.

In one alternative, to further enhance the growth of nanostructures ofcarbon and the like, nickel and iron salts, such as nickel chloride oriron chloride, or other sources of nickel or iron, may be added to thecoal powder in the initial mixing step.

As sis temperature continue to increase, in the 800 C to 1500 C range,another important transformation takes place. The different claymaterials such as kaolinite and illite in the coal start to melt and/ordecompose. These clays decompose into nano sized particles of SiO2(silica) and Al2O3 (alumina). These are the primary constituents of flyash which is a troublesome by product of the combustion of coal.However, in this technology, the silica and alumina nanoparticleschemically bond/alloy into a ceramic structure through silicon andoxygen bonds formed by the transformation of the PDC resin into aceramic. In the final phases of sis, most, if not all, of the remaininghydrogen and sulfur are volatized and escape. The resulting nanocomposite can be viewed as a blend of nanoporous polymer derived ceramicand carbon, such as graphene sheets, tubes, fullerenes and the like,which bond to domains of nanoporous graphitized carbon that has beentoughened by the in situ growth of nanotubes, nanofibers and the like.The in situ nucleation of alumina and silica nanoparticles adds to thecomplex and remarkable structure and mechanical.

In one example, pure PDC resins shrink by 15% to 25% when pyrolyzed. Ifpure coal was fed, it would shrink by a similar amount, depending on thegrade of coal used. In one example, a batch of coal/PDC resin is mixedof one type and in various proportions with a different batch made witha different PDC resin. As an example, a batch of anthracite coal with aPDC resin for silicon oxycarbide ceramic may be mixed with 70 wt %anthracite coal and 30 wt % silicon oxycarbide precursor. A second batchof bituminous coal and a PDC resin for silicon carbide ceramic may beformed by mixing 60 wt % bituminous coal with 40 wt % silicon carbideprecursor. Then, the two batches are mixed together in whatever ratio isdesired, such as a 50/50 blend of the two batches, comprising a ceramicnanocomposite that has 50% silicon carbide containing and 50% siliconoxycarbide containing micro domains blended homogenously together.Nanocomposites may be bi-scale porous with micro scale pores at theinter-coal-particle scale (between the coal particles) and nanoporous atthe intra-coal-particle scale (inside each coal particle). Therefore,each pores may be infiltrated with additional PDC resin of the same ordifferent family, encapsulating in situ nanotubes and nanofibers withinmicropores and nanopores. In one example, the nanocomposite is pyrolyzeda second time, and the new ceramic forms in the pores with improvedstrength and toughness from reinforcing nanostructures, such as tubesand fibers.

In addition to proppants, complex composite particles having a sizerange of 0.01 micrometers to 5 millimeters are used in a variety ofapplications. For example, when added to polymer-derived ceramics, thesecomplex composite particles improve hardness, strength, oxidationresistance and decrease cost. Fillers other than coal dust may beincluded such as silica, clay, SiC, boron carbide, glass powder, glassmicrobeads, silica sand, aluminum hydroxide, magnesium hydroxide,magnesium carbonate, or other metallic carbonates, phosphates, orsilicates, and the like. In one example, a sub-bituminous coal is addedand substantially reduces costs while maintaining similar properties.Coal dust is added in at least 1 wt % of the complex composite particle,which enhances properties due to certain chemical reactions, and fillersand constituents may be added to control other properties of thecomposite, such as coefficient of thermal expansion and the like.Preferably, at least 20 wt % is coal dust, more preferably, at least 40wt %, such that a substantial fraction of complex particles include insitu growth of fullerenes, nanotubes and/or nanofibers. In one example,a different ceramic forming polymer (such as one with a higherviscosity) is utilized to provide a “seal-coat” on the outer shell ofthe complex composite particle. For example, this may keep water out ofthe pores and decrease the effective density, as well as toughen theparticle or proppant.

In one example, an organic resin with controlled viscosity is used as a“seal coat” on the particle to keep water out and improve toughness,such as an epoxy, vinyl ester, polyester, polyimide, polyamide,silicone-modified phenolic resin, acrylic, pmma, styrene,dicyclopentadiene, divinyl benzene. For example, an inorganic resin,such as silicone resins or siloxane type resins, provide a seal coatmaterial. In another example, an inorganic resin coating that iscomposed of one or more of the following materials may be used: aperoxide cured silicone or siloxane resin; a platinum cured silicon orsiloxane resin; an amine cured silicone or siloxane resin; a silicone orsiloxane cured using an organo-metallic catalyst such as a tin, zinc, orother metal based catalyst. In yet another example, an inorganic resincoating where the silicone or siloxane resin is modified with an organicresin monomer or polymer to improve crosslinking density or otherproperties is used. The organic monomer or polymer may include one ormore of the following, for example: dicyclopentadiene, divinylbenzene,epoxy, polyimide, polyamide, phenolic resin, phenol, formaldehyde,styrene, vinyl ester, any organic diene.

In one example, a proppant or particle comprises coal or coal derivedmaterial and a polymer or resin matrix. A polymer or resin matrix may becomprised of phenolic resin (phenol-formaldehyde, or any modification ofphenolic type resin); epoxy resin or any modified epoxy resin includingsiloxane or phenolic modified resins; vinyl ester resins or anyderivatives of vinyl ester resins; polyester resins or any derivativesof polyester resins; dicyclopentadiene or any organic diene based resin;divinylbenzene type resins or any divinylbenzene modified resin; anyresin modified with dicyclopentadiene or any cyclic dienes. In oneexample, a proppant or particle comprises a thermoplastic resin, such aspolymethylmethacrylate (PMMA), polycarbonate, toughened polycarbonate,PEEK, melamine or melamine modified resins including melamine modifiedphenolic resins. A proppant or particle may be comprised of a polymer orresin matrix that is the resultant polymer created/or synthesized by thereaction of one or more monomer or short polymer chain siloxanes orsilanes with organic monomers or polymers. The organic monomer orpolymer reacted with the siloxane or silanes may include one or more ofthe following: dicyclopentadiene, divinylbenzene, epoxy, polyimide,polyamide, phenolic resin, phenol, formaldehyde, styrene, vinyl ester,any organic diene. A proppant or particle may comprise a polymer orresin matrix of one or more of the following materials: a peroxide curedsilicone or siloxane resin; a platinum cured silicon or siloxane resin;an amine cured silicone or siloxane resin; a silicone or siloxane curedusing an organo-metallic catalyst such as a tin, zinc, or other metalbased catalyst. The silicone or siloxane resin may be modified by eithermixing or reacting the silicone or siloxane resin with an organic resinmonomer or polymer to improve crosslinking density or other properties.The organic monomer or polymer may include one or more of the following:dicyclopentadiene, divinylbenzene, epoxy, polyimide, polyamide, phenolicresin, phenol, formaldehyde, styrene, vinyl ester, any organic diene,for example. Alternatively, the silicone or siloxane resin is modifiedby either mixing or reacting the silicone or siloxane resin with otherinorganic monomers or polymers such astetravinyltetramethylcyclotetrasiloxane, tetramethylcyclotetrasiloxane,phenyl silane, diphenylsilane, vinyl-terminated polydimethylsiloxane,hydride terminated polydimethylsiloxane, or hydroxy-terminatedpolydimethylsiloxne

In one example, a proppant or particle may be comprised of coal orcoal-derived material in a polymer or resin matrix that is cured orprocessed at a temperature of 15° C. to 600° C. preferably in the rangeof 75° C. to 300° C.

For example, a proppant or particle may be comprised of coal orcoal-derived material in a polymer or resin matrix utilized as in theextraction of hydrocarbon materials from the earth or used as acomponent of equipment that is used in the extraction of hydrocarbonmaterials from the earth.

This detailed description provides examples including features andelements of the claims for the purpose of enabling a person havingordinary skill in the art to make and use the inventions recited in theclaims. However, these examples are not intended to limit the scope ofthe claims, directly. Instead, the examples provide features andelements of the claims that, having been disclosed in thesedescriptions, claims and drawings, may be altered and combined in waysthat are known in the art.

What is claimed is:
 1. A complex composite particle comprises a core comprised of a first material, wherein the first material is a pyrolyzed composite made of a coal dust derived from coal and a polymer precursor that forms a polymer derived ceramic material, the coal dust and the polymer precursor that forms the polymer derived ceramic, when pyrolyzed, being mixed together to form a mixture, the mixture being pyrolyzed in a substantially non-oxidizing atmosphere and transforming into a ceramic matrix composite, wherein the coal dust of the core is preheated at a temperature no greater than 400 degrees centigrade, prior to being mixed with the polymer precursor, to produce an in situ additive comprising carbon and organic compounds that vaporize and react when pyrolyzed in the presence of the polymer precursor, after being mixed with the polymer precursor, and the mixture of the in situ additive and the polymer precursor form the pyrolyzed composite during pyrolyzation.
 2. The complex composite particle of claim 1, further comprising a coating substantially enclosing the core within the coating.
 3. The complex composite particle of claim 2, wherein the coating is comprised of a polymer derived ceramic material, whereby the coating may be comprised of the same polymer derived ceramic material as the polymer derived ceramic material of the core or a different polymer derived ceramic material than the polymer derived ceramic material of the core.
 4. The complex composite particle of claim 3, wherein the polymer derived ceramic material of the core is a silicon-oxy-carbide polymer derived ceramic material.
 5. The complex composite particle of claim 4, wherein the polymer derived ceramic material of the coating is the same as the polymer derived ceramic material of the core.
 6. The complex composite particle of claim 2, wherein the polymer derived ceramic material of the coating is pyrolyzed in a substantially non-oxidizing atmosphere such that a hard shell substantially encloses the core.
 7. The complex composite particle of claim 6, wherein the core comprises substantially no carbon particles after pyrolyzation of the core.
 8. The complex composite particle of claim 6, wherein the core comprises carbon particles chemically or physically bonded to a ceramic matrix after pyrolyzation of the core.
 9. The complex composite particle of claim 8, wherein the carbon particles are chemically bonded to the ceramic matrix.
 10. The complex composite particle of claim 2, wherein the coating is partially dried until tacky.
 11. The complex composite particle of claim 2, wherein an initial layer of the coating is pyrolyzed and a subsequent layer of the coating is added to the surface of the initial layer, wherein the subsequent layer is a liquid, slurry or tacky material.
 12. The complex composite particle of claim 11, wherein the subsequent layer is partially dried until tacky.
 13. The complex composite particle of claim 1, wherein the hydraulic diameter is 3 to 5 microns.
 14. The complex composite particle of claim 1, wherein the complex composite particle is combined as building blocks for composite structures or beads made of a plurality of the complex composite particle. 